It should not be getting hotter than 50C. If it is, the lifespan will be shortened, possibly dramatically.

When I use drives in my Vantec dock, they're only spun up (generating significant heat) while the backup is in progress. I spin them down and/or remove power when not actively backing things up.

Here in the Great White North, this means the drive doesn't normally get very hot.

If you're running Linux, you can use hddtemp to interrogate the actual drive temperature from its onboard sensor, assuming an eSATA connection.

Cheers